Visible light communications (VLC) is a data communication method wherein data is embedded in the light output of a light source emitting visible light (usually in the wavelength range of 375-780 nm). In VLC, it is desirable to use light emitting diodes (LEDs) as a light source, as the LEDs are able to achieve a considerably higher data transmission rate compared to fluorescent lamps.
Coded light has been proposed as a special VLC technique to enable advanced control of light sources and to transmit information using these light sources. Since the primary purpose of a light source is typically illumination, it is important that the modulation of the light output of the light source is invisible to human eyes, also referred to as a flicker-free modulation. Different methods have been developed in the prior art to achieve a flicker-free modulation of the light output, wherein amplitude or intensity modulation of the light output of the light source is one of the most commonly applied methods. To embed data in this method, the light source is either switched fully switched on/off (on-off keying) or the amplitude is modified. This method typically needs to be combined with a coding method to reduce the spectral content in the frequencies below several hundreds of Hertz, which are visible to human eyes. Due to this coding, the effective data rate is reduced and a higher bandwidth needs to be applied to achieve the same data rate. This might be challenging, since LEDs and their associated drivers are typically limited in their bandwidth.
A further method which is applied in the prior art for VLC is frequency modulation, or frequency shift keying. Here, the data is embedded by switching between two different modulation (switching) frequencies, where e.g. one frequency signifies the “zero” bit and the second frequency the “one” bit. A disadvantage of this method is that the applied frequencies need to be high to avoid visible flicker, which reduces the effective bandwidth available for data communication.
A even further modulation method in the prior art for VLC modulation uses multiple light sources with a different color point (x,y-coordinates in a chromaticity diagram) to embed data in the light. In this method, the perceived intensity of the light is kept constant, while the color is changed. This is based on the understanding that the human eye is less sensitive to color point changes than to intensity changes. However, a disadvantage of this method is that the modulation amplitude can only be limited, since an unwanted visibility will otherwise occur. Alternatively, the modulation frequency needs to be high, which may also be undesired. Further, the light source used need to have sufficiently different color points with respect to each other (e.g. red, green, blue), which might not be desired for a general illumination, e.g. white light.
In view of this, there is a wish to provide an alternative modulation of the light which overcomes the above-mentioned disadvantages of the modulation methods in the prior art.